Drax and Gamora are almost a necessity to a fight against Thanos, due to insider information about him; and if they're both there, there's no reason that the whole Guardians team wouldn't be there, too.

I'd say it's a reasonably safe bet that the Guardians will at least cameo in TA2.

There are so many characters that it's kind of a problem. Not every character can appear in the Avengers because then the film ends up being unfocused and risks not being as good.